1. Field of the Invention
The present invention relates to a positive type photosensitive composition, the solubility of which, in an aqueous alkaline solution, is enhanced by being exposed to infrared rays. More specifically, the invention relates to a positive type photosensitive composition useful as a recording layer of a planographic printing plate precursor that can be subjected to the so-called direct plate-making method, according to which a printing plate can be directly made up by scanning of an infrared laser on the basis of digital signals received from a computer or the like.
2. Description of the Related Art
Hitherto, various photosensitive compositions have been used for the formation of visible images, or as a material for planographic printing plate precursors. Recently, remarkable developments have taken place in the area of lasers for planographic printing. With particular, regard to both solid lasers and semiconductor lasers having an emission wavelength within a near infrared or infrared wavelength range, it has become easy to obtain lasers which have a high level of output and which are at the same time compact. These lasers are extremely useful as light sources for exposure when printing plates are made up directly on the basis of digital data received from a computer or the like.
Materials which can be used for positive type planographic printing plate precursors applicable for infrared lasers include, as essential components, a binder resin soluble in an aqueous alkaline solution (hereinafter referred to where appropriate as an “alkali-soluble resin”), and an infra red dye which absorbs light to generate heat. When an image is formed in a positive type planographic printing plate precursor, the infra red dye interacts with the binder resin in its unexposed portions (image portions) so as to function as a dissolution inhibitor which can substantially reduce the solubility of the binder resin. On the other hand, in its exposed portions (non-image portions), interaction of the infra red dye with the binder resin is weakened by the heat generated. Consequently, an exposed portion can turn into a state in which it can be dissolved in an alkaline developer, so that an image is formed thereon.
However, insofar as infrared-laser-applicable positive planographic printing plate precursor materials are concerned, differences in the degree of resistance against dissolution in a developer between unexposed portions (image portions) and exposed portions (non-image portions) therein, that is, differences in development latitude have not yet been sufficient under various conditions of use. Thus, problems have occurred insofar that, with changes in conditions of use of materials, materials have tended to be either excessively developed or inadequately developed.
Such problems stem from fundamental differences in plate-making mechanisms between infrared-laser-applicable positive type planographic printing plate precursor materials and positive type planographic printing plate precursor materials from which printing plates are made up by exposure to ultra violet rays.
In other words, positive type planographic printing plate precursor materials from which printing plates are made up by exposure to ultra violer rays each include, as essential components, a binder resin soluble in an aqueous alkaline solution and an onium salt, or a quinonediazide compound. This onium salt or quinonediazide compound not only interacts with the binder resin in unexposed portions (image portions) to function as a dissolution inhibitor, but in exposed portions (non-image portions) it is also decomposed by light and generates an acid to function as a dissolution promoter. In this way, the onium salt, or the quinonediazide compound, performs dual functions.
On the other hand, in infrared-laser-applicable positive type planographic printing plate precursor materials, the infra red dye functions only as a dissolution inhibitor of unexposed portions (image portions), and does not promote the dissolution of exposed portions (non-image portions). Furthermore, heat generated in exposed portions near the interface between the photosensitive layer and the support diffuses to the support. Thus, in some cases heat may not be effectively used for forming images. As a result, the materials of this type have a drawback insofar that a film residue is easily generated. Accordingly, demands have been made for infrared-laser-applicable positive type planographic printing plate precursor materials which have both a high sensitivity and a wide development latitude.
As solutions to these problems, in order to maintain the rate of the residual film in image portions low and to increase the degree of contrast between image portions and non-image portions, the following examples have been disclosed: an example wherein an organic acid such as polyacrylic acid is also used in a positive type photosensitive composition including a photothermal conversion material and an alkali-soluble resin (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 10-282643); an example wherein a methyl methacrylate-(meth)acrylic acid copolymer is also used in the same composition (see, for example, JP-A No. 2001-324808); and other examples. However, these examples are have proved inadequate in the terms of compatibility between the image-formability of the compositions (in particular, alkali resistance of image portions, which should not be too high) and the storage stability thereof.
A technique wherein a recording layer is made into a multilayered structure, thereby improving the stability of the layer has also been suggested. A positive type image forming material has been disclosed which has a lower layer including a sulfonamide-containing polymer, and an upper layer containing a resin having a phenolic hydroxyl group and an infrared absorbent (see, for example, JP-A No. 11-218914). However from the viewpoint of the printing resistance of image portions, there is still scope for improvement.
A technique has also been suggested in which a lower layer includes an alkali-soluble binder and an upper layer includes an epoxy compound and a curing agent, the upper layer being cross-linked by being heated up to a temperature of about 100° C. at the time when it is applied and dried, thereby improving the alkali resistance of the layers (see, for example, EP patent No. 960728). Since under this system the epoxy compound and the curing agent are made to coexist in the upper layer, a cross-linking reaction is generated only in the upper layer, at the time that it is being dried after application. Thus, the depths of the recording layer which has been cured are insufficient and the printing resistance can accordingly not be improved. Moreover, in this case, improvements in printing resistance by the burning treatment cannot be obtained.